In electrophotography, a photoconductive surface is charged in the dark and then subjected to a light image of the document which is to be reproduced to form a latent electrostatic image corresponding to the original document. The latent electrostatic image thus formed can be developed by a dry developer powder which can be transferred to plain paper and then fused thereon by heat. This requires energy and entails the use of a higher voltage outlet than is normally found in offices. Because of the disadvantages of having to fuse a dry developed image, many electrophotographic copiers use liquid developers. These comprise an insulating liquid through which are disseminated toner particles. The photoconductor on which the developed image resides has been wetted by the liquid developer. The amount of liquid on the photoconductor can be reduced by a reverse roller such as shown in Hayashi et al U.S. Pat. No. 3,907,423. This reduces the quantity of liquid remaining on the photoconductive surface before the developed image is transferred to plain paper. The amount of liquid, however, is still such that liquid-developer machines frequently require a slight amount of heat to evaporate this residue of the liquid, which carries the toner particles, from the paper to which it has been transferred. Electrophotographic apparatus using a light hydrocarbon liquid as a carrier may be operated at a higher speed than if the carrier liquid were a higher-boiling hydrocarbon such as a light mineral oil. Evaporation of hydrocarbons in the atmosphere is disadvantageous if machines are operated for long periods of time in enclosed spaces. With a higher-boiling carrier liquid, if substantially all of the carrier liquid can be removed from the image, leaving it just moist enough so it can be transferred to a carrier sheet such as paper or the like, an electrophotographic machine employing a liquid developer would have many advantages.
It has been attempted to remove the excess liquid from the surface of the photoconductor after development of the image, both by a foam closed-cell elastomeric roller in which the open pores or pockets tend to blot up the liquid and by squeegeeing the excess liquid remaining on the photoconductive surface after the latent electrostatic image has been developed. Both of these disciplines, however, have resulted in streaking, tailing or blurring, so that the image becomes unsharp and unsatisfactory, and thus have failed to achieve their purpose.
1. Field of the Invention
Our invention relates to a method and apparatus for removing excess developing liquid from photoconductive surfaces, after a latent electrostatic image has been developed by a liquid developer, while producing sharp and pleasing reproductions of the original document which has been electrophotocopied.
2. Description of the Prior Art
Seanor et al U.S. Pat. No. 3,959,574 discloses a biasable polyurethane roller adapted to be positioned behind a carrier sheet to effect transfer of the developed latent electrostatic image from a photoconductive surface to paper. No suggestion of the use of this roller in any liquid-developer system is disclosed, nor is there any suggestion that the roller may be used to remove excess liquid from a liquid-developed electrostatic image.
Smith et al U.S. Pat. No. 3,955,533 shows a freely-rotatable polyurethane squeegee roller for removing excess developer liquid from a liquid-developed latent electrostatic image on a photoconductive drum. This system smears or blurs the image and possesses disadvantages which our invention overcomes.
Hayashi et al U.S. Pat. No. 3,907,423 discloses a reverse roller for metering the quantity of liquid remaining on the photoconductive surface after a latent electrostatic image has been developed. The reverse roller does not contact the photoconductive surface, and hence there remains a layer of liquid developer on the photoconductive surface, which it is the object of the instant invention to remove without distorting or blurring or disfiguring the developed image. While Hayashi et al disclose a biasing of the reverse roller, it is biased in the wrong direction and for the wrong purpose in respect of the instant invention. In Hayashi et al, the potential is biased so as to remove toner in the background areas, or non-image areas, from the surface of the photoconductor so that there will be no background or gray areas in the transferred image.
Buckley et al U.S. Pat. No. 3,863,603 discloses a roll carrying a conductive polyurethane coating which is intended for use in forming a magnetic brush in a dry developing process for latent electrostatic images.
Hudson U.S. Pat. No. 3,807,853 discloses the use of a closed-cell polyurethane foam formed with pores or pockets for cleaning a photoconductive surface of dry particles.
Matkan U.S. Pat. No. 3,752,119 discloses a squeegee roller for removing excess liquid from the photoconductive drum after a latent electrostatic image has been developed. Matkan shows nothing not shown by Smith et al.
Riley U.S. Pat. No. 3,656,200 discloses a sponge-like roller of polyurethane foam for cleaning the photoconductive drum after a liquid-developed electrostatic image has been transferred to a carrier sheet.
Hunstiger U.S. Pat. No. 3,384,051 relates to a method of moving a paper coated with a photoconductor through a liquid development bath and then squeegeeing the development liquid from the photoconductive surface of the paper after the image has been developed.
Matsumoto et al U.S. Pat. No. 3,368,526 is similar to Hunstiger in that it squeegees excess developing liquid from a flexible sheet bearing a developed electrostatic image.